From Concept to Reality (part two)

We finally finished our first set of master molds, so we can cast the production molds and see how well they work. Making these production molds is a fairly straightforward process. Just mix the two-part silicone and pour it into the master molds.

The two halves of the production molds matched up perfectly! We were a little worried that they wouldn’t because this is the first time we have ever tried making a mold from two completely separate pieces. Our next step was to test them by making casts of the actual pieces.

The test cast didn’t go as well as planned. It revealed a few problem that I had suspected, but hoped wouldn’t be a big deal. The resin didn’t flow properly into the corner of the lower left portion of the casting. This was because that portion was just too thin to enable the resin to flow easily. The capstone on the end post also didn’t fill properly.

The first problem was solved by increasing the thickness of that portion. Unfortunately that requires the lower half to be the same thickness as the upper half. I had been very fond of the look of having the railing overhang the superstructure of the bridge. I liked the extra depth that it added, but having flatter sides is a necessary compromise. The second problem is much simpler and can be alleviated by adding more channels for air to escape through. If that didn’t fix it we would thicken the cap piece as well.

After making the required changes to the virtual model, we printed another master mold. The changes are fairly subtle, but if you look close they are easy to spot. A new production mold was then made using the new master, and we did another test pour of the resin to see if the problems were fixed.

This test pour went a lot better than the first, with the biggest problem of the lower left corner being fixed. The extra channels on the end post cap helped, but didn’t fix it; the next step is to make the cap thicker. This will require making changes to both halves of the mold so it involves starting over the printing process with both sides this time, and not just one.